Offload sync function calls to a thread

[Rifa-111]

Summary

Sync tool functions registered with FastMCP were called directly on the
asyncio event loop, blocking it for the duration of the call. Any sync
tool performing I/O (e.g. using requests, time.sleep, file reads)
would stall all other concurrent tasks on the same server.

Change

In call_fn_with_arg_validation (src/mcp/server/fastmcp/utilities/func_metadata.py),
wrap sync calls with anyio.to_thread.run_sync() instead of calling
them directly:

Before:

if fn_is_async:
    return await fn(**arguments_parsed_dict)
else:
    return fn(**arguments_parsed_dict)

After:

if fn_is_async:
    return await fn(**arguments_parsed_dict)
else:
    return await anyio.to_thread.run_sync(lambda: fn(**arguments_parsed_dict))

This is consistent with how FastAPI handles sync route handlers, and how
FileResource and DirectoryResource already handle blocking reads
within this codebase.

anyio.to_thread.run_sync() uses copy_context() internally so
contextvars propagate correctly to the worker thread. The MCP Context
object is unaffected as it is passed explicitly via
arguments_to_pass_directly.

Testing

Added a test that registers a sync tool using time.sleep and asserts
that two concurrent calls complete in roughly the time of one sleep,
confirming they run in parallel without blocking the event loop.

Closes #1839

[cubic-dev-ai]

1 issue found across 1 file

Prompt for AI agents (unresolved issues)

Check if these issues are valid — if so, understand the root cause of each and fix them. If appropriate, use sub-agents to investigate and fix each issue separately.


<file name="src/mcp/server/fastmcp/utilities/func_metadata.py">

<violation number="1" location="src/mcp/server/fastmcp/utilities/func_metadata.py:96">
P1: Add the missing `anyio` import before using `anyio.to_thread.run_sync`; otherwise every synchronous tool call fails at runtime.</violation>
</file>

<sub>Reply with feedback, questions, or to request a fix.

Fix all with cubic | Re-trigger cubic</sub>

[cubic-dev-ai]

P1: Add the missing anyio import before using anyio.to_thread.run_sync; otherwise every synchronous tool call fails at runtime.

Prompt for AI agents

Check if this issue is valid — if so, understand the root cause and fix it. At src/mcp/server/fastmcp/utilities/func_metadata.py, line 96:

<comment>Add the missing `anyio` import before using `anyio.to_thread.run_sync`; otherwise every synchronous tool call fails at runtime.</comment>

<file context>
@@ -93,7 +93,11 @@ async def call_fn_with_arg_validation(
             return await fn(**arguments_parsed_dict)
         else:
-            return fn(**arguments_parsed_dict)
+            return await anyio.to_thread.run_sync(lambda: fn(**arguments_parsed_dict))
+            # Sync functions are offloaded to a thread to avoid blocking the event loop.
+            # anyio.to_thread.run_sync() uses copy_context() internally, so contextvars
</file context>